Self-healing additive technology

ABSTRACT

A novel gas generant is formed by integrating an additive or compound selected from polymeric compounds, long-chain hydrocarbons, long-chain fluorocarbons, and paraffinic compounds in or about the gas generant. A gas generating composition is formed in a typical manner, and the additive may be either homogeneously mixed within the gas generating composition, or, it may simply be coated over the gas generant.

CROSS REFERENCE TO RELATED APPLICATIONS/PATENTS

The present application claims the benefit of U.S. Provisional Application No. 61/732,206 having a filing date of Nov. 30, 2012.

The present application contemplates and describes various compositions such as gas generating compositions that contain phase stabilized ammonium nitrate (PSAN). U.S. Pat. Nos. 5,872,329, 6,074,502, 6,210,505, 5,545,272, 5,531,941, and 6,306,232 are each incorporated by reference in their entirety. Each of these patents describe, but do not limit, exemplary compositions Containing phase stabilized ammonium nitrate. The following description exemplifies a group of compositions containing phase stabilized ammonium nitrate as described in U.S. Pat. No. 6,074,502, but with improvements as described below.

FIELD OF THE INVENTION

The present invention relates to nontoxic gas generating compositions which upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates, but that also, upon combustion, exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.

The present invention yet further relates to the propensity for gas generants containing PSAN to be fortified with an additive that permits self-correction or “self-healing” of potential anomalies in the gas generant as it is thermally cycled between a relatively higher temperature and a relatively lower temperature. During these temperature changes, sometimes compositions containing phase stabilized ammonium nitrate may still be hampered by slight changes in phase that over time may affect the repeatability of performance of gas generating compositions containing PSAN.

BACKGROUND OF THE INVENTION

The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the teachings of which are hereby incorporated by reference in their entirety.

In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.

One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.

The use of phase stabilized ammonium nitrate is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107° C. The compositions must also retain structural integrity when cycled between −40° C. and 107° C.

Often, gas generant compositions incorporating phase stabilized or pure ammonium nitrate exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NO_(x) for example, depending on the composition of the associated additives such as plasticizers and binders. In addition, ammonium nitrate contributes to poor ignitability, lower burn rates, and performance variability. Several known gas generant compositions incorporating ammonium nitrate utilize well known ignition aids such as BKNO₃ to solve this problem. However, the addition of an ignition aid such as BKNO₃ is undesirable because it is a highly sensitive and energetic compound, and furthermore, contributes to thermal instability and an increase in the amount of solids produced.

Certain gas generant compositions comprised of ammonium nitrate are thermally stable, but have burn rates less than desirable for use in gas inflators. To be useful for passenger restraint inflator applications, gas generant compositions generally require a burn rate of at least 0.4 inch/second (ips) or more at 1000 psi. Gas generants with burn rates of less than 0.40 ips at 1000 psi do not ignite reliably and often result in “no-fires” in the inflator.

Yet another concern that must be addressed is that the U.S. Department of Transportation (DOT) regulations require “cap testing” for gas generants. Because of the sensitivity to detonation of fuels often used in conjunction with ammonium nitrate, most propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator.

Many nonazide gas generants burn at temperatures well-above known azide-based gas generants. To simplify cooling requirements, a nonazide gas generant composition suitable for use in an airbag inflator would be an improvement.

Finally, gas generant compositions as disclosed in co-owned U.S. Pat. Nos. 5,872,329 and 6,306,232 are suitable for use within an automotive airbag inflator. However, certain combustion characteristics respective to certain gas generant compositions can be improved. For example, compositions containing PSAN, nitroguanidine, and a nonmetal salt of a tetrazole are disadvantaged by a shortened burn time and a higher combustion temperature as compared to the compositions of the present invention.

During temperature cycling inside of an inflator, tablets or wafers of gas generating compositions containing phase-stabilized ammonium nitrate or PSAN (e.g. PSAN containing about 85 to 90 weight percent ammonium nitrate coprecipitated with about 10-15 weight percent of a potassium salt such as potassium nitrate), may lose density especially in the presence of moisture or humidity. It is believed that in some circumstances, the density loss may lead to less predictable performance criteria.

Certain compositions containing PSAN have also included adsorbents such as 13× molecular sieve, thereby addressing the moisture concerns by minimizing or eliminating the ambient moisture. It would be an improvement in the art to manage the moisture concerns while concurrently addressing other concerns including manufacturing improvements, for example.

DESCRIPTION OF THE RELATED ART

A description of related art follows, the complete teachings of which are herein incorporated by reference.

U.S. Pat. No. 5,545,272 to Poole discloses the use of gas generant compositions consisting of nitroguanidine (NQ), at a weight percent of 35%-55%, and phase stabilized ammonium nitrate (PSAN) at a weight percent of 45%-65%. NQ, as a fuel, is preferred because it generates abundant gases and yet consists of very little carbon or oxygen, both of which contribute to higher levels of CO and NOx in the combustion gases. According to Poole, the use of phase stabilized ammonium nitrate (PSAN) or pure ammonium nitrate is problematic because many gas generant compositions containing the oxidizer are thermally unstable. Poole has found that combining NQ and PSAN in the percentages given results in thermally stable gas generant compositions. However, Poole reports burn rates of only 0.32-0.34 inch per second, at 1000 psi. As is well known, burn rates below 0.4 inch per second at 1000 psi are simply too low for confident use within an inflator.

In U.S. Pat. No. 5,531,941 to Poole. Poole teaches the use of PSAN, and two or more fuels selected from a specified group of nonazide fuels. Poole adds that gas generants using ammonium nitrate (AN) as the oxidizer are generally very slow burning with burning rates at 1000 psi typically less than 0.1 inch per second. He further teaches that for air bag applications, burning rates of less than about 0.4 to 0.5 inch per second are difficult to use. The use of PSAN is taught as desirable because of its propensity to produce abundant gases and minimal solids, with minimal noxious gases. Nevertheless, Poole recognizes the problem of low burn rates and thus combines PSAN with a fuel component containing a majority of TAGN, and if desired one or more additional fuels. The addition of TAGN increases the burn rate of ammonium nitrate mixtures. According to Poole, TAGN/PSAN compositions exhibit acceptable burn rates of 0.59-0.83 inch/per second. TAGN, however, is a sensitive explosive that poses safety concerns in processing and handling. In addition, TAGN is classified as “forbidden” by the Department of Transportation, therefore complicating raw material requirements.

In U.S. Pat. No. 5,500,059 to Lund et al., Lund states that burn rates in excess of 0.5 inch per second (ips) at 1,000 psi, and preferably in the range of from about 1.0 ips to about 1.2 ips at 1,000 psi, are generally desired. Lund discloses gas generant compositions comprised of a 5-aminotetrazole fuel and a metallic oxidizer component. The use of a metallic oxidizer reduces the amount of gas liberated per gram of gas generant, however, and increases the amount of solids generated upon combustion.

The gas generant compositions described in Poole et al, U.S. Pat. Nos. 4,909,549 and 4,948,439, use tetrazole or triazole compounds in combination with metal oxides and oxidizer compounds (alkali metal, alkaline earth metal, and pure ammonium nitrates or perchlorates) resulting in a relatively unstable generant that decomposes at low temperatures. Significant toxic emissions and particulate are formed upon combustion. Both patents teach the use of BKNO₃ as an ignition aid.

The gas generant compositions described in Poole, U.S. Pat. No. 5,035,757, result in more easily filterable solid products but the gas yield is unsatisfactory.

Chang et al, U.S. Pat. No. 3,954,528, describes the use of TAGN and a synthetic polymeric binder in combination with an oxidizing material. The oxidizing materials include pure AN although, the use of PSAN is not suggested. The patent teaches the preparation of propellants for use in guns or other devices where large amounts of carbon monoxide, nitrogen oxides, and hydrogen are acceptable and desirable. Because of the practical applications involved, thermal stability is not considered a critical parameter.

Grubaugh, U.S. Pat. No. 3,044,123, describes a method of preparing solid propellant pellets containing AN as the major component. The method requires use of an oxidizable organic binder (such as cellulose acetate, PVC. PVA, acrylonitrile and styrene-acrylonitrile), followed by compression molding the mixture to produce pellets and by heat treating the pellets. These pellets would certainly be damaged by temperature cycling because commercial ammonium nitrate is used, and the composition claimed would produce large amounts of carbon monoxide.

Becuwe, U.S. Pat. No. 5,034,072, is based on the use of 5-oxo-3-nitro-1,2,4-triazole as a replacement for other explosive materials (HMX, RDX, TATB, etc.) in propellants and gun powders. This compound is also called 3-nitro-1,2,4-triazole-5-one (“NTO”). The claims appear to cover a gun powder composition which includes NTO, AN and an inert binder, where the composition is less hygroscopic than a propellant containing ammonium nitrate. Although called inert, the binder would enter into the combustion reaction and produce carbon monoxide making it unsuitable for air bag inflation.

Lund et al, U.S. Pat. No. 5,197,758, describes gas generating compositions comprising a nonazide fuel which is a transition metal complex of an aminoarazole, and in particular are copper and zinc complexes of 5-aminotetrazole and 3-amino-1,2,4-triazole which are useful for inflating air bags in automotive restraint systems, but generate excess solids.

Wardle et al, U.S. Pat. No. 4,931,112, describes an automotive air bag gas generant formulation consisting essentially of NTO (5-nitro-1,2,4-triazole-3-one) and an oxidizer wherein said formulation is anhydrous.

Ramnarace, U.S. Pat. No. 4,111,728, describes gas generators for inflating life rafts and similar devices or that are useful as rocket propellants comprising ammonium nitrate, a polyester type binder and a fuel selected from oxamide and guanidine nitrate. Ramnarace teaches that ammonium nitrate contributes to burn rates lower than those of other oxidizers and further adds that ammonium nitrate compositions are hygroscopic and difficult to ignite, particularly if small amounts of moisture have been absorbed.

Bucerius et al, U.S. Pat. No. 5,198,046, teaches the use of diguanidinium-5,5′-azotetrazolate (GZT) with KNO₃ as an oxidizer, for use in generating environmentally friendly, non-toxic gases. Bucerius teaches away from combining GZT with any chemically unstable and/or hygroscopic oxidizer. The use of other amine salts of tetrazole such as bis-(triaminoguanidinium)-5,5′-azotetrazolate (TAGZT) or aminoguanidinium-5,5′-azotetrazolate are taught as being much less thermally stable when compared to GZT.

Boyars, U.S. Pat. No. 4,124,368, describes a method for preventing detonation of ammonium nitrate by using potassium nitrate.

Mishra, U.S. Pat. No. 4,552,736, and Mehrotra et al, U.S. Pat. No. 5,098,683, describe the use of potassium fluoride to eliminate expansion and contraction of ammonium nitrate in transition phase.

Chi, U.S. Pat. No. 5,074,938, describes the use of phase stabilized ammonium nitrate as an oxidizer in propellants containing boron and as useful in rocket motors.

In U.S. Pat. No. 5,125,684 to Cartwright, an extrudable propellant for use in crash bags is described as comprising an oxidizer salt, a cellulose-based binder and a gas generating component. Cartwright also teaches the use of “at least one energetic component selected from nitroguanidine (NG), triaminoguanidine nitrate, ethylene dinitramine, cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN) . . . ”

In U.S. Pat. No. 4,925,503 to Canterbury et al, an explosive composition is described as comprising a high energy material, e.g., ammonium nitrate and a polyurethane polyacetal elastomer binder, the latter component being the focus of the invention. Canterbury also teaches the use of a “high energy material useful in the present invention . . . preferably one of the following high energy materials: RDX, NTO, TNT, HMX, TAGN, nitroguanidine, or ammonium nitrate.”

Hass, U.S. Pat. No. 3,071,617, describes long known considerations as to oxygen balance and exhaust gases.

Stinecipher et al, U.S. Pat. No. 4,300,962, describes explosives comprising ammonium nitrate and an ammonium salt of a nitroazole.

Prior, U.S. Pat. No. 3,719,604, describes gas generating compositions comprising aminoguanidine salts of azotetrazole or of ditetrazole.

Poole, U.S. Pat. No. 5,139,588, describes nonazide gas generants useful in automotive restraint devices comprising a fuel, an oxidizer and additives.

Hendrickson, U.S. Pat. No. 4,798,637, teaches the use of bitetrazole compounds, such as diammonium salts of bitetrazole, to lower the burn rate of gas generant compositions. Hendrickson describes burn rates below 0.40 ips, and an 8% decrease in the burn rate when diammonium bitetrazole is used.

Chang et al, U.S. Pat. No. 3,909,322, teaches the use of nitroaminotetrazole salts with oxidizers such as pure ammonium nitrate. HMX, and 5-ATN. These compositions are used as gun propellants and gas generants for use in gas pressure actuated mechanical devices such as engines, electric generators, motors, turbines, pneumatic tools, and rockets. In contrast to the amine salts disclosed by Hendrickson, Chang teaches that gas generants comprised of 5-aminotetrazole nitrate and salts of nitroaminotetrazole exhibit burn rates in excess of 0.40 ips. On the other hand, Chang also teaches that gas generants comprised of HMX and salts of nitroaminotetrazole exhibit burn rates of 0.243 ips to 0.360 ips. No data is given with regard to burn rates associated with pure AN and salts of nitroaminotetrazole.

Highsmith et al, U.S. Pat. No. 5,516,377, teaches the use of a salt of 5-nitraminotetrazole. NQ, a conventional ignition aid such as BKNO₃, and pure ammonium nitrate as an oxidizer, but does not teach the use of phase stabilized ammonium nitrate. Highsmith states that a composition comprised of ammonium nitraminotetrazole and strontium nitrate exhibits a burn rate of 0.313 ips. This is to low for automotive application. As such, Highsmith emphasizes the use of metallic salts of nitraminotetrazole.

Poole et al., U.S. Pat. No. 5,386,775, teaches the use of low energy fuels including hydrazodicarbonamide and azodicarbonamide to reduce the combustion temperature of a propellant. However. Poole states that it is necessary to use an alkali metal salt of an organic acid to obtain an acceptable burn rate. This would create higher levels of solids.

Onishi et al, U.S. Pat. No. 5,439,251, teaches the use of a tetrazole amine salt as an air bag gas generating agent comprising a cationic amine and an anionic tetrazolyl group having either an alkyl with carbon number 1-3, chlorine, hydroxyl, carboxyl, methoxy, aceto, nitro, or another tetrazolyl group substituted via diazo or triazo groups at the 5-position of the tetrazole ring. The inventive thrust is to improve the physical properties of tetrazoles with regard to impact and friction sensitivity, and therefore does not teach the combination of an amine or nonmetal tetrazole salt with any other chemical.

Lund et al, U.S. Pat. No. 5,501,823, teaches the use of nonazide anhydrous tetrazoles, derivatives, salts, complexes, and mixtures thereof, for use in air bag inflators. The use of bitetrazole-amines, not amine salts of bitetrazoles, is also taught.

SUMMARY OF THE INVENTION

The aforementioned concerns are solved by a nonazide gas generant for a vehicle passenger restraint system containing phase stabilized ammonium nitrate, one or more primary nonazide fuels, and one or more additives selected from polymer or long-chain hydrocarbons or long-chain fluorocarbons including paraffinic compounds such as paraffin, synthetic paraffin, polyethylene, and mixtures thereof.

The preferred primary nonazide fuels may be selected from a group including nitroaromatics such as ammonium salt of 3,5-dinitrosalicylic acid (ADNSA) or 3,5-dinitrosalicylic acid (DNSA).

Preferred secondary fuels may be selected from tetrazole-containing compounds such as 5,5′-bitetrazole or 5,5′-bis-1H-tetrazole, salts of 5,5′-bitetrazole or 5,5′-bis-1H-tetrazole such as diammonium salt of 5,5′-bis-1H-tetrazole or diammonium bitetrazole, diguanidinium-5,5′-azotetrazolate (GZT), and nitrotetrazoles such as 5-nitrotetrazole; triazoles such as nitroaminotriazole, nitrotriazoles, and 3-nitro-1,2,4 triazole-5-one; and salts of tetrazoles and triazoles.

One or more secondary fuel(s) may also be selected from amine and other nonmetal salts of tetrazoles and triazoles having a nitrogen containing cationic component and a tetrazole and/or triazole anionic component. The anionic component comprises a tetrazole or triazole ring, and an R group substituted on the 5-position of the tetrazole ring, or two R groups substituted on the 3- and 5-positions of the triazole ring. The R group(s) is selected from hydrogen and any nitrogen-containing compounds such as amino, nitro, nitramino, tetrazolyl and triazolyl groups. The cationic component is formed from a member of a group including amines, aminos, and amides including ammonia, hydrazine, guanidine compounds such as guanidine, aminoguanidine, diaminoguanidine, triaminoguanidine, dicyandiamide, nitroguanidine, nitrogen substituted carbonyl compounds such as urea, carbohydrazide, oxamide, oxamic hydrazide, bis-(carbonamide)amine, azodicarbonamide, and hydrazodicarbonamide, and, amino azoles such as 3-amino-1,2,4-triazole, 3-amino-5-nitro-1,2,4-triazole, 5-aminotetrazole and 5-nitraminotetrazole. Optional inert additives such as clay, alumina, or silica may be used as a binder, slag former, coolant or processing aid. Optional ignition aids comprised of nonazide propellants may also be utilized in place of conventional ignition aids such as BKNO₃.

In accordance with the present invention, one or more additives are selected from polymer or long-chain hydrocarbons or long-chain fluorocarbons including paraffinic compounds such as paraffin, synthetic paraffin, polyethylene, polytetrafluoroethylene (PTFE), and mixtures thereof. The additive may be homogeneously mixed within the composition as known in the art, or, the additive may be imparted onto finished gas generant in whatever form the gas generant is provided in. For example, tablets or pellets may be dipped into a warm bath of paraffin or some other additive as described, and then allowed to dry after the dipping process. The paraffin may, for example, be dry-blended with the granulated main propellant in a plow mixer or melted onto the main propellant dry powder in a fluidized bed dryer. The propellant/paraffin mix is then pressed into a tablet. An exemplary spray-dried composition formed from the ammonium salt of 3,5-dinitrosalicylic acid, diammonium salt of 5,5′-bis-1H-tetrazole, and phase stabilized ammonium nitrate, such as a composition formed as described in U.S. patent application Ser. No. 13,955,905, herein incorporated by reference in its entirety, may have paraffin melted over the granules and then have the mixture dried in a fluidized bed dryer for example. The currently preferred method is to dry the granules in the fluidized bed dryer, then dry mix the paraffin and dry granules in a plow mixer at room temperature. The exemplary composition is then formed into a pellet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view showing the general structure of an inflator in accordance with the present invention.

FIG. 2 is a schematic representation of an exemplary vehicle occupant restraint system containing a gas generant composition in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A nonazide gas generant comprises phase stabilized ammonium nitrate (PSAN), one or more primary nonazide high-nitrogen fuels, and if desired, one or more secondary nonazide high-nitrogen fuels. An exemplary spray-dried composition is formed from the ammonium salt of 3,5-dinitrosalicylic acid, diammonium salt of 5,5′-bis-1H-tetrazole, and phase stabilized ammonium nitrate. Paraffin may then be dry-mixed with the dried spray-dried composition, or, paraffin may be melted over the composition granules, and then dried in a known manner, such as within a fluidized bed dryer for example. In yet another embodiment, the secondary or tertiary nonazide fuel may for example, be selected from the group including azodicarbonamide (ADCA) and hydrazodicarbonamide (AH), and mixtures thereof. In general, the paraffin may be added to any formulation, but it is preferably used with compositions containing one or more nitroaromatics, one or more tetrazole-based aromatics, and phase-stabilized ammonium nitrate.

The preferred primary nonazide fuels may be selected from a group including nitroaromatics such as ammonium salt of 3,5-dinitrosalicylic acid (ADNSA) or 3,5-dinitrosalicylic acid (DNSA).

One or more secondary nonazide high-nitrogen fuels are selected from a group including tetrazoles and bitetrazoles such as 5-nitrotetrazole and 5,5′-bitetrazole or 5,5′-bis-1H-tetrazole (BHT), and salts of 5,5′-bitetrazole or 5,5′-bis-1H-tetrazole such as diammonium salt of 5,5′-bis-1H-tetrazole (BHT-2NH3) or diammonium bitetrazole; triazoles and nitrotriazoles such as nitroaminotriazole and 3-nitro-1,2,4 triazole-5-one; nitrotetrazoles; and salts of tetrazoles and salts of triazoles. More specifically, salts of tetrazoles include in particular, amine, amino, and amide nonmetal salts of tetrazole and triazole selected from the group including monoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.1GAD), diguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2GAD), monoaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.1AGAD), diaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT.2AGAD), monohydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT.1HH), dihydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT.2HH), monoammonium salt of 5,5′-bis-1H-tetrazole (BHT.1NH₃), diammonium salt of 5,5′-bis-1H-tetrazole (BHT.2NH₃), mono-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT.1ATAZ), di-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT.2ATAZ), and diguanidinium salt of 5,5′-Azobis-1H-tetrazole (ABHT.2GAD).

Amine salts of triazoles include monoammonium salt of 3-nitro-1,2,4-triazole (NTA.1NH₃), monoguanidinium salt of 3-nitro-1,2,4-triazole (NTA.1GAD), diammonium salt of dinitrobitriazole (DNBTR.2NH₃), diguanidinium salt of dinitrobitriazole (DNBTR.2GAD), and monoammonium salt of 3,5-dinitro-1,2,4-triazole (DNTR.1NH₃).

An exemplary and generic nonmetal salt of tetrazole as shown in Formula I includes a cationic nitrogen containing component, Z, and an anionic component comprising a tetrazole ring and an R group substituted on the 5-position of the tetrazole ring. A generic nonmetal salt of triazole as shown in Formula II includes a cationic nitrogen containing component. Z, and an anionic component comprising a triazole ring and two R groups substituted on the 3- and 5-positions of the triazole ring, wherein R₁ may or may not be structurally synonymous with R₂. An R component is selected from a group including hydrogen or any nitrogen-containing compound such as an amino, nitro, nitramino, or a tetrazolyl or triazolyl group as shown in Formula I or II, respectively, substituted directly or via amine, diazo, or triazo groups. The compound Z is substituted at the 1-position of either formula, and is formed from a member of the group comprising amines, aminos, and amides including ammonia, carbohydrazide, oxamic hydrazide, and hydrazine; guanidine compounds such as guanidine, aminoguanidine, diaminoguanidine, triaminoguanidine, dicyandiamide and nitroguanidine; nitrogen substituted carbonyl compounds or amides such as urea, oxamide, bis-(carbonamide)amine, azodicarbonamide, and hydrazodicarbonamide; and, amino azoles such as 3-amino-1,2,4-triazole, 3-amino-5-nitro-1,2,4-triazole, 5-aminotetrazole, 3-nitramino-1,2,4-triazole, 5-nitraminotetrazole, and melamine.

In accordance with the present invention, a preferred gas generant composition results from the mixture of one or more primary nonazide high-nitrogen fuels comprising 5%-45%, and more preferably 9%-27% by weight of the gas generant composition; one or more secondary nonazide high-nitrogen fuels comprising 1%-35%, and more preferably 1%-15% by weight of the gas generant composition; and PSAN comprising 55%-85%, and more preferably 66%-78% by weight of the gas generant composition. Tetrazoles are more preferred than triazoles due to a higher nitrogen and lower carbon content thereby resulting in a higher burning rate and lower carbon monoxide. Salts of tetrazoles are even more preferred because of superior ignition stability. As taught by Onishi, U.S. Pat. No. 5,439,251, herein incorporated by reference, salts of tetrazoles are much less sensitive to friction and impact thereby enhancing process safety. Nonmetallic salts of bitetrazoles are more preferred than nonmetallic salts of tetrazoles due to superior thermal stability. As also taught by Onishi, nonmetallic salts of bitetrazoles have higher melting points and higher exothermal peak temperatures thereby resulting in greater thermal stability when combined with PSAN. The diammonium salt of bitetrazole is most preferred because it is produced in large quantities and readily available at a reasonable cost.

Accordingly, an exemplary gas generating composition contains phase-stabilized ammonium nitrate (preferably stabilized by co-precipitating 10-15 wt % of a potassium salt such as potassium nitrate with 85-90 wt % of ammonium nitrate) at about 70-80 wt % of the total weight of the gas generating composition. An ammonium salt of 3,5-dinitrosalicylic acid is provided at about 10-15 wt % as a primary fuel, and diammonium salt of 5,5′-bis-1H-tetrazole is provided at about 10-15 wt % as a secondary fuel, said weight percents taken with regard to the total weight of the gas generating composition. A preferred composition contains about 76.6 wt % of phase stabilized ammonium nitrate (PSAN), about 13.4 wt % of the ammonium salt of 3,5-dinitrosalicylic acid, and about 10.0 wt % of a diammonium salt of 5,5′-bis-1H-tetrazole. A paraffinic compound or a similar polymeric compound in accordance with the present invention is added as a percent of the total mass of the gas generating composition being coated. Exemplary manufacturers of paraffinic or polymeric additives in accordance with the present invention include for example the Munzing corporation (located in Germany) marketing the paraffin under the trade names CERETAN MT 8415 and CERETAN MX 9820. Other manufacturers include Micro Powders (located in New York) selling the additives under the trade names MP-22XXF, MP-620XXF, and MicroKlear 418.

In accordance with procedures well known in the art, the foregoing primary and secondary nonazide fuels are blended with an oxidizer such as PSAN. The manner and order in which the components of the gas generant compositions of the present invention are combined and compounded is not critical so long as the proper particle size of ingredients are selected to ensure the desired mixture is obtained. The compounding is performed by one skilled in the art, under proper safety procedures for the preparation of energetic materials, and under conditions that will not cause undue hazards in processing nor decomposition of the components employed. For example, the materials may be wet blended, or dry blended and attrited in a ball mill or Red Devil type paint shaker and then pelletized by compression molding. The materials may also be ground separately or together in a fluid energy mill, sweco vibroenergy mill or bantam micropulverizer and then blended or further blended in a v-blender prior to compaction. Alternatively, the compositions may be spray-dry formed as detailed in U.S. application Ser. No. 13/955,905 to form homogeneous gas generating compositions, prior to adding one or more additives selected from polymer or long-chain hydrocarbons or long-chain fluorocarbons including paraffinic compounds such as paraffin, synthetic paraffin, polyethylene, and mixtures thereof. Plow mixers may be used to mix dry gas generating composition granules or powder, homogeneously mixed with the paraffinic or other polymeric additives as stated above.

Compositions having components more sensitive to friction, impact, and electrostatic discharge should be wet ground separately followed by drying. The resulting fine powder of each of the components may then be wet blended by tumbling with ceramic cylinders in a ball mill jar, for example, and then dried. Less sensitive components may be dry ground and dry blended at the same time.

Phase stabilized ammonium nitrate may be prepared as taught in co-owned U.S. Pat. No. 5,531,941 entitled, “Process For Preparing Azide-free Gas Generant Composition”. Other nonmetal inorganic oxidizers such as ammonium perchlorate, or oxidizers that produce minimal solids when combined and combusted with the fuels listed above, may also be used. The ratio of oxidizer to fuel is preferably adjusted so that the amount of oxygen allowed in the equilibrium exhaust gases is less than 3% by weight, and more preferably less than or equal to 2% by weight. The oxidizer may comprise 55%-85% by weight of the gas generant composition.

The gas generant constituents of the present invention are commercially available. For example, the amine salts of tetrazoles may be purchased from Toyo Kasei Kogyo Company Limited, Japan. As secondary fuels, azodicarbonamide and hydrazodicarbonannide may be obtained for example from Nippon Carbide in Japan, or from Aldrich Chemical Co., Inc. in Milwaukee, Wis. The components used to synthesize PSAN, as described herein, may be purchased from Fisher or Aldrich. Triazole salts may be synthesized by techniques, such as those described in U.S. Pat. No. 4,236,014 to Lee et al.; in “New Explosives: Nitrotriazoles Synthesis and Explosive Properties”, by H. H. Licht, H. Ritter, and B. Wanders, Postfach 1260, D-79574 Weil am Rhein; and in “Synthesis of Nitro Derivatives of Triazoles”, by Ou Yuxiang, Chen Boren, Li Jiarong, Dong Shuan, Li Jianjun, and Jia Huiping, Heterocycles, Vol. 38, No. 7, pps. 1651-1664, 1994. The teachings of these references are herein incorporated by reference. Other compounds in accordance with the present invention may be obtained as taught in the references incorporated herein, or from other sources well known to those skilled in the art.

An optional burn rate modifier, from 0-10% by weight in the gas generant composition, is selected from a group including an alkali metal, an alkaline earth or a transition metal salt of tetrazoles or triazoles; an alkali metal or alkaline earth nitrate or nitrite; TAGN; dicyandiamide, and alkali and alkaline earth metal salts of dicyandiamide; alkali and alkaline earth borohydrides; or mixtures thereof. An optional combination slag former and coolant, in a range of 0 to 10% by weight, is selected from a group including clay, silica, glass, and alumina, or mixtures thereof. When combining the optional additives described, or others known to those skilled in the art, care should be taken to tailor the additions with respect to acceptable thermal stability, burn rates, and ballistic properties.

In accordance with the present invention, the combination of PSAN, one or more primary nonazide high-nitrogen fuels, and one or more secondary nonazide high-nitrogen fuels as determined by gravimetric procedures, yields beneficial gaseous products equal to or greater than 90% of the total product mass, and solid products equal to or lesser than 10% of the total product mass. Fuels suitable in practicing the present invention are high in nitrogen content and low in carbon content thereby providing a high burn rate and a minimal generation of carbon monoxide.

The synergistic effect of the high-nitrogen fuels, in combination with an oxidizer producing minimal solids when combined with the fuels, results in several long-awaited benefits. Increased gas production per mass unit of gas generant results in the use of a smaller chemical charge. Reduced solids production results in minimized filtration needs and therefore a smaller filter. Together, the smaller charge and smaller filter thereby facilitate a smaller gas inflator system. Furthermore, the gas generant compositions of the present invention have burn rates and ignitability that meet and surpass performance criteria for use within a passenger restraint system, thereby reducing performance variability.

Table 1 compares certain compositions containing PSAN. As shown, compositions containing PSAN typically have a high combustion temperature. PSAN10 indicates ammonium nitrate stabilize with 10% by weight potassium nitrate.

TABLE 1 Combustion Temp, at Composition Source 3000 psi (K) 70.46% PSAN10, 16.54% BHT- Example 2 2078 2NH3, and 13.00% ADCA 67.17% PSAN10, 19.83% BHT- U.S. application 2188 2NH3, and 13.00% NQ Ser. No. 08/851,503 58.2% PSAN10, and 41.8% NQ Poole 5,534,272 2423 Example 4 64.70% PSAN15, 31.77% TAGN, Poole 5,531,941 2278 and 3.53% oxamide Example 7

TABLE 2 Tank Peak Burn- Pres. at Tank out Max. Composition Source 10 ms Pressure Time Slope 70.46% PSAN10 Example 2 27 kPa 178 kPa 51 ms  6.3 kPa/ms 16.54% BHT-2NH3 13.00% ADCA 67.17% PSAN10 U.S. 69 kPa 183 kPa 30 ms 10.3 kPa/ms 19.83% Pat. No. BHT-2NH3 6,306,232 13.00% NQ

To optimize passenger kinematics, it is sometimes desirable that an inflator slowly generate gas during the initial stages of airbag deployment. After an initial slow onset, the inflator must then quickly and completely fill the airbag to provide adequate occupant restraint. In practice, combining a slow inflation onset with a high gas output is difficult at best. One known method combines a dual chamber system within a single inflator. As taught in co-owned U.S. Pat. No. 6,306,232, the addition of nitroguanidine (NQ) to PSAN-based formulations provides tailoring of the ballistic curve as described above. However, certain nitroguanidine-based PSAN compositions tend to burn out too quickly.

TABLE 3 Pressure Composition Source Pressure Range Exponent 70.46% PSAN10, Example 2 0-2200 psi 0.83 16.54% 2200-5000 psi   0.21 BHT-2NH3, and 13.00% ADCA 66.34% PSAN10 Example 3 0-5000 psi 0.53 and 33.66% ADCA 59.0% PSAN10, Poole Not Available 0.47 and 41.0% NQ 5,545,272; Example 1

Most propellants follow the equation R_(b)=aP^(n) where R_(b) is the linear burn rate, P is pressure, and a and n are constants. The constant n is known as the pressure exponent and characterizes the dependence of the propellant burn rate on pressure. As described by Chi in U.S. Pat. No. 5,074,938 (incorporated herein by reference), the pressure exponent should be as close to zero as possible. As n increases, a very small change in pressure will result in a large change in the burn rate. This could result in high performance or ballistic variability, or over-pressurization. Therefore, for automotive airbag applications, a pressure exponent at about 0.30 or less is desired over the operating pressure of the inflator. Although most burn rates are reported at 1000 psi (6.9 Mpa), the actual operating pressure in most inflators is above 2200 psi.

In accordance with the present invention, it has been discovered that paraffinic or polymeric additives as described herein provide seal-healing of propellants containing phase-stabilized ammonium nitrate. In particular, after thermal shock and heat aging, it has been found that the ballistic behavior of the propellants of the present invention maintain a consistent ballistic profile. It is believed that the paraffinic or polymeric additive imparts a flexibility to the propellant notwithstanding the potentially slight phase change characteristics of ammonium nitrate. Additionally, it has unexpectedly been discovered that the use an additive such as paraffin or a long-chain polymer improves the flow properties of PSAN-based compositions when forming the tablets thereby ensuring the integrity of the finished propellant. Accordingly, the paraffinic or polymeric additives of the present invention function as a lubricant to reduce the friction when compressed tablets are ejected from a die during the manufacturing process.

The present invention is illustrated by the following examples. All compositions are given in percent by weight.

Example 1 Spray Congealing Method

A feed tank was prepared containing an aqueous slurry of phase stabilized ammonium nitrate wherein the following constituents were added in the amounts indicated:

ADNSA is provided at about 13.4 percent; ammonium nitrate (66.6%) and potassium nitrate (10.0%) to form PSAN at about 76.6%; diammonium salt of bitetrazole (BHT-2NH3) at about 10%; and ammonium carbonate at about 0.3% by weight of the other constituents combined. The feed tank temperature was maintained at about 240 F and the feed tank pressure was about 20 psi. The atomizing air pressure was about 50 psi. The air flow was about 800 cubic feet per minute in a counter current spray cooling tower. The air temperature of the spray cooling tower was maintained at about 120F. The process produced substantial amounts of homogeneously mixed, spherical shaped granules that have proven to be easier to manufacture into grains, as compared to an equivalent composition made by a wet method.

Example 2 Spray Congealing Method

A feed tank was prepared containing an aqueous slurry of phase stabilized ammonium nitrate wherein the following constituents were added in the amounts indicated:

ADNSA is provided at about 13.4 percent; ammonium nitrate (66.6%) and potassium nitrate (10.0%) to form PSAN at about 76.6%; diammonium salt of bitetrazole (BHT-2NH3) at about 10%; and ammonium carbonate at about 0.3% by weight of the other constituents combined. The feed tank temperature was maintained at about 250F and the feed tank pressure was about 23 psi. The atomizing air pressure was about 40 psi. The air flow was about 600-800 cubic feet per minute in a counter current spray cooling tower. The air temperature of the spray cooling tower was maintained at about 120F. The process produced substantial amounts of homogeneously mixed, spherical shaped granules that have proven to be easier to manufacture into grains, as compared to an equivalent composition made by a wet method.

Example 3 Wet Mix Method Including a Secondary Fuel

A composition was made by providing a jacketed mixing vessel containing about two liters of ethanol. To this solution, about 753 grams of dinitrosalicylic acid (DNSA) was added while continuously stirring. The solution was then heated slowly to about 105 C over about thirty minutes and maintained throughout the remaining process. Once the DNSA was completely dissolved, about 4352 grams of ammonium nitrate, about 122 grams of potassium nitrate, about 227 grams of potassium carbonate (whereby potassium nitrate and potassium carbonate taken together provide a potassium source for phase stabilization of the ammonium nitrate), about 595 grams of diammonium bitetrazole, and one liter of water are added together into the vessel, while continuously and mechanically stirring. A bright yellow precipitate forms immediately in a viscous, paint-like consistency. After about one hour, the mix forms crumbly solids. The mixing and heating is continued until the desired dryness is obtained. If desired, the mix may be formed into desired shapes such as pellets or tablets and then dried to a desired moisture content, in an oven for example. The process produced substantial amounts of multi-shaped granules that were bimodal in particle size distribution. Although useful, the composition did not flow as well in pelletizing methods as compared to the products of Examples 1 and 2.

Example 4

Five compositions without paraffin were formed by the method of Example 1. As stated in weight percents of the total composition, the first composition contained 65.00 wt % of ammonium nitrate, 9.70 wt % of potassium nitrate, and 15.30 wt % of ADNSA; no paraffin was added. A second composition contained 64.94 wt % of ammonium nitrate, 9.69 wt % of potassium nitrate, 9.99 wt % BHT-2NH3, and 15.28 wt % of ADNSA; 0.10 wt % of paraffin was homogeneously dry-mixed into the granules of the second composition. A third composition contained 64.87 wt % of ammonium nitrate, 9.68 wt % of potassium nitrate, 9.98 wt % of BHT-2NH3, and 15.27 wt % of ADNSA; 0.20 wt % of paraffin was mixed into the granules of the third composition. A fourth composition contained 64.68 wt % of ammonium nitrate, 9.65 wt % of potassium nitrate, 9.95 wt % of BHT-2NH3, and 15.22 wt % of ADNSA; 0.50% of paraffin was mixed into the granules of the fourth composition. A fifth composition contained 64.35 wt % of ammonium nitrate, 9.60 wt % of potassium nitrate, 9.90 wt % of BHT-2NH3, and 15.15 wt % of ADNSA; 1.00% of paraffin was mixed into the granules of the fifth composition.

In accordance with the present invention, the wet spray congealed powder resulting from the process of Example 1 was blended with paraffin and then placed in a fluidized bed dryer and is dried above the paraffin melting point to coat the granules. In general, a vibratory fluid bed may be used that vibrates during operation at a preferred drying temperature of about 175 F. Alternatively, dried spray congealed powder resulting from the process of Example 1 may be blended or mixed with paraffin by employing a plow mixer, ball mixer, ribbon blender, or other mixer as known in the art, to generally result in a homogeneous mix of the spray congealed composition and the paraffin (or other desired additive in accordance with the present invention). Again, one preferred process is drying the spray congeal granules in a vibratory fluid bed, then dry-coating the dry granules with paraffin in a plow mixer.

Example 5

The ejection force of propellant tablets pressed from a standard propellant mold was evaluated with regard to the five compositions of Example 4. Composition 1 had an ejection force of about 1360 N. Composition 2 had an ejection force of about 580 N. Composition 3 had an ejection force of about 480 N. Composition 4 had an ejection force of about 400 N. Composition 5 had an ejection force of about 380 N. The ease of ejection is indicative of the ease of manufacturing of the present compositions, while yet maintaining the structural integrity of the propellant or gas generating composition tablet.

Example 6

The change in ejection force required to eject the tablets from the mold of Example 5 was evaluated, under the same operating conditions of Example 5. Accordingly, five compositions were again formed as in Example 1. As in Example 4, a lubricant was added to the five compositions. However, in this case, boron nitride instead of paraffin was added at 0.00 wt % (composition 6), 0.25 wt % (composition 7), 0.50 wt % (composition 8) and 1.0 wt % (composition 9) of the total composition after integration or mixing of the boron nitride, respectively. The change in force required to extrude the compressed or finished tablet from the mold with regard to composition 6 was as follows: composition 7 resulted in a 29% reduction in force; composition 8 resulted in a 46% reduction in force; and composition 9 resulted in a 62% reduction in force.

Example 7

The change in ejection force required to eject the tablets from the mold of Example 5 was evaluated, under the same operating conditions of Example 5. Accordingly, five compositions were again formed as in Example 1. As in Example 4, a lubricant was added to the five compositions. However, in this case, graphite instead of paraffin was added at 0.00 wt % (composition 10), 0.25 wt % (composition 11), 0.50 wt % (composition 12) and 1.0 wt % (composition 13) of the total composition after integration or mixing of the graphite, respectively. The change in force required to extrude the compressed or finished tablet from the mold with regard to composition 10 was as follows: composition 11 resulted in a 38% reduction in force; composition 12 resulted in a 52% reduction in force; and composition 13 resulted in a 65% reduction in force.

Example 8

The change in ejection force required to eject the tablets from the mold of Example 5 was evaluated, under the same operating conditions of Example 5. Accordingly, five compositions were again formed as in Example 1. As in Example 4, paraffin was added to the five compositions at 0.00 wt % (composition 1), 0.2 wt % (composition 3), 0.50 wt % (composition 4) and 1.0 wt % (composition 5) of the total composition after integration or mixing of the paraffin, respectively. The change in force required to extrude the compressed or finished tablet from the mold with regard to composition 1 was as follows: composition 3 resulted in a 65% reduction in force; composition 4 resulted in a 70% reduction in force; and composition 5 resulted in a 73% reduction in force. As shown in Examples 6-8, the addition of paraffin provided substantially improved reductions in ejection force as compared to compositions not containing paraffin and as compared to other lubricants of Examples 6 and 7.

Example 9

The density of the compositions of Example 4 was evaluated: composition 1 had a density of about 1.6585 g/cc; composition 2 had a density of about 1.658 g/cc; composition 3 had a density of about 1.656 g/cc; composition 4 had a density of about 1.654 g/cc; and composition 5 had a density of about 1.6425 g/cc.

Example 10

The hydrophilicity or hydrophobicity of the compositions of Example 4 were evaluated. It is desired that compositions repel moisture to mitigate any potentially adverse effects of the hygroscopic ammonium nitrate nature of the present compositions. Stated another way, it is desired to mitigate the moisture uptake of a composition over time. Moisture uptake may result in variations of ballistic performance. It is desired that the predictability of ballistic performance be maintained while maximizing the other benefits of the present compositions. The surface energy of a solid can be determined by measuring the contact angle of a water droplet with the surface of the material. In general, the lower the contact angle between the water and the material, then the greater the surface energy of the material. For purposes of this example only, a surface with a relatively lower contact angle (less than 90 degrees) was determined to be hydrophilic; a surface with a relatively higher contact angle (greater than 90 degrees) was determined to be hydrophobic.

Using a pipette, two drops or about 0.1 ml of water was placed on the surface of each of the compositions of Example 4. Composition 1 indicated hydrophilicity with a contact angle of about 30 degrees. Composition 2 indicated hydrophilicity with a contact angle of about 45 degrees. Composition 3 indicated hydrophilicity with a contact angle of about 60 degrees. Composition 4 indicated a neutral relative moisture uptake with a contact angle of about 90 degrees. Composition 5 indicated hydrophobicity with a contact angle of about 135 degrees. Accordingly, this example indicates hydrophobic characteristics of propellants containing PSAN and greater relative amounts of paraffin (or other additives in accordance with the present invention) at or above 0.5 wt %.

Example 11

The ballistic behavior of the compositions of Example 4 were evaluated in a known manner by evaluating the pressure curves over time within a known inflator actuated within a 60 L tank. In general, the chamber pressures and the tank pressures from the beginning of combustion to 0.1 seconds were substantially equivalent, indicating that the desired ballistic properties were maintained with the addition of a paraffinic or polymeric additive in accordance with the present invention.

Example 12 Comparative Example

A mixture of ammonium nitrate (AN), potassium nitrate (KN), and guanidine nitrate (GN) was prepared having 45.35% NH₄NO₃, 8.0% KN, and 46.65% GN. The ammonium nitrate was phase stabilized by coprecipitating with KN at 70-90 degrees Celsius.

The mixture was dry-blended and ground in a ball mill. Thereafter, the dry-blended mixture was compression-molded into pellets. The burn rate of the composition was determined by measuring the time required to burn a cylindrical pellet of known length at constant pressure. The burn rate at 1000 pounds per square inch (psi) was 0.257 inches per second (in/sec); the burn rate at 1500 psi was 15.342 in/sec. The corresponding pressure exponent was 0.702.

Example 13 Comparative Example

A mixture of 52.20% NH₄NO₃, 9.21% KN, 28.59% GN, and 10.0% 5-aminotetrazole (5AT) was prepared and tested as described in Example 1. The burn rate at 1000 psi was 0.391 in/sec and the burn rate at 1500 psi was 0.515 in/sec. The corresponding pressure exponent was 0.677.

Example 14 Comparative Example

Table 4 illustrates the problem of thermal instability when typical nonazide fuels are combined with PSAN:

TABLE 4 Thermal Stability of PSAN - Non-Azide Fuel Mixtures Non-Azide Fuel(s) Combined with PSAN Thermal Stability 5-aminotetrazole (5AT) Melts with 108 C. onset and 116 C. peak. Decomposed with 6.74% weight loss when aged at 107 C. for 336 hours. Poole ′272 shows melting with loss of NH₃ when aged at 107 C. Ethylene diamine dinitrate, Poole ′272 shows melting at less than nitroguanidine (NQ) 100 C. 5AT, NQ Melts with 103 C. onset and 110 C. peak. 5AT, NQ quanidine nitrate Melts with 93 C. onset on 99 C. peak. (GN) GN, NQ Melts with 100 C. onset and 112 C. Decomposed with 6.49% weight loss when aged at 107 C. for 336 hours. GN, 3-nitro-1,2,4-triazole Melts with 108 C. onset and 110 C. peak. (NTA) NQ, NTA Melts with 111 C. onset and 113 C. peak. Aminoguanidine nitrate Melts with 109 C. onset and 110 C. peak. 1H-tetrazole (1HT) Melts with 109 C. onset and 110 C. peak. Dicyandiamide (DCDA) Melts with 114 C. onset and 114 C. peak. GN, DCDA Melts with 104 C. onset and 105 C. peak. NQ, DCDA Melts with 107 C. onset and 115 C. peak. Decomposed with 5.66% weight loss when aged at 107 C. for 336 hours. 5AT, GN Melts with 70 C. onset and 99 C. peak. Magnesium salt of 5AT Melts with 100 C. onset and 111 C. peak. (M5AT)

In this example, “decomposed” indicates that pellets of the given formulation were discolored, expanded, fractured, and/or stuck together (indicating melting), making them unsuitable for use in an air bag inflator. In general, any PSAN-nonazide fuel mixture with a melting point of less than 115 C will decompose when aged at 107 C. As shown, many compositions that comprise well-known nonazide fuels and PSAN are not fit for use within an inflator due to poor thermal stability.

Example 15 Comparative Example

A mixture of 56.30% NH₄NO₃, 9.94% KN, 17.76% GN, and 16.0% 5AT was prepared and tested as described in Example 1. The burn rate at 1000 psi was 0.473 in/sec and the burn rate at 1500 psi was 0.584 in/sec. The corresponding pressure exponent was 0.518. The burn rate is acceptable, however, compositions containing GN, 5-AT, and PSAN are not thermally stable as shown in Table 4, EXAMPLE 3.

For Examples 8-10, the phase stabilized ammonium nitrate contained 10% KN (PSAN10) and was prepared by co-crystallization from a saturated water solution at 80 degrees Celsius. The diammonium salt of 5,5′-bis-1H-tetrazole (BHT-2NH₃), hydrazodicarbonamide (AH), and azodicarbonamide (ADCA) were purchased from an outside supplier.

Example 16

A composition was prepared containing 76.52% PSAN10, 13.48% BHT-2NH3, and 10.00% AH. Each material was dried separately at 105 degrees Celsius. The dried materials were then mixed together and pulverized to a homogeneous powder with a mortar and pestle. The mixture was tested using a differential scanning calorimeter (DSC) and found to melt at about 156 degrees Celsius. The composition was also tested using a thermogravimetric analyzer (TGA) and found to have a 91.8% gas conversion and no mass loss until about 185 degrees Celsius. The DSC and TGA results demonstrate the high thermal stability and high gas yield of this composition.

Example 17

A composition was prepared containing 70.46% PSAN10, 16.54% BHT-2NH3, and 13.00 ADCA. Each material was dried separately at 105 degrees Celsius. The dried materials were then mixed together and tumbled with alumina cylinders in a large ball mill jar. After separating the alumina cylinders, the final product resulted in 1500 grams of homogeneous and pulverized powder. The powder was formed into granules to improve flow properties, and then compression molded into pellets (0.184″ diameter, 0.090″ thick) on a high speed tablet press.

The composition was tested using a DSC and found to melt at about 155 degrees Celsius. The composition was also tested using a TGA and found to have a 91.8% gas conversion and no mass loss until about 170 degrees Celsius. The DSC and TGA results demonstrate the excellent thermal stability and high gas yield of the composition.

The composition has a burn rate at 1000 psi of 0.45 inches per second (ips). As shown in FIG. 2, the burn rate follows the equation R_(b)=0.00143P^(0.834) from 0 psi to about 2200 psi, and R_(b)=0.163P^(0.213) from about 2200 psi to about 5000 psi. The burn rate data demonstrate that compositions using both the primary and secondary fuels in conjunction with PSAN have both a desirable burn rate (greater than 0.40 ips at 1000 psi) and pressure exponent (less than 0.30 from about 2200-5000 psi.)

The tablets formed on the high speed press were loaded into an inflator and fired inside a 60 L tank. The ballistic performance showed an acceptable gas output and burnout time along with a low onset and slope.

Example 18 Comparative Example

A composition was prepared containing 66.34% PSAN10, and 33.66% ADCA. Each material was dried separately at 105 degrees Celsius. The dried materials were then mixed together and tumbled with alumina cylinders in a small ball mill jar. After separating the alumina cylinders, the final product resulted in 75 grams of homogeneous and pulverized powder.

The mixture was tested using a DSC and found to melt at about 155 degrees Celsius. The composition was also tested using a TGA and found to have a 93.5% gas conversion and no mass loss until about 164 degrees Celsius. The DSC and TGA results demonstrate the excellent thermal stability and high gas yield of this composition.

The composition had a burn rate at 1000 psi of 0.31 inches per second (ips). As shown in FIG. 3, the burn rate follows the equation R_(b)=0.00770P^(0.535) over the entire 0-5000 psi range. The burn rate data demonstrate that compositions using only the secondary fuel in conjunction with PSAN have an insufficient burn rate (less than 0.40 ips at 1000 psi) and an excess pressure exponent over the desired operating pressure (greater than 0.30 from about 2200-5000 psi).

In yet another aspect of the present invention, all compositions referred to herein may contain an additive described as a polymer or long-chain hydrocarbon or fluorocarbon including paraffinic compounds such as paraffin, synthetic paraffin, polyethylene, and mixtures thereof. More preferably, the compositions contain such polymeric or long-chain hydrocarbon or fluorocarbon additives in about 0.01 to 1.0% by weight of the composition, and more preferably at about 0.1% to 0.5% by weight of the composition. The lubricity imparted by the paraffin aids in the pressing and processing of the various propellants. Furthermore, paraffinic compounds homogeneously or heterogeneously mixed within the PSAN-based compositions exemplified herein, or coated with paraffin through a dipping process for example, have been found to impart a hydrophobic quality to the surface of the propellant thereby minimizing any water uptake due to the typically hygroscopic nature of PSAN-based compositions.

Additives such as the paraffinic compounds and polymeric compounds may be provided by known suppliers such as IGI Wax of Titusville, Pa.; Hase Petroleum Wax Co. of Arlington Heights, Ill.; and Lintech International of Macon, Ga. for example. The additive may be provided to enable mixing with other granulated constituents for example, and the additive may be mixed into the composition by well known methods.

Paraffin may be advantageously employed in compositions of the present invention by adding it at about 0.1 to 1.0 wt % of the total composition. Preferably, the paraffinic or polymeric additive is employed at about 0.2 to 1 parts per 100 parts of gas generating composition (without the paraffinic or polymeric additive).

As shown in FIG. 1, an exemplary inflator or gas generating system 10 incorporates a primary gas generating composition 12 formed as described herein, wherein the inflator may be manufactured as known in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety.

Referring now to FIG. 2, the exemplary inflator or gas generating system 10 described above may also be incorporated into an airbag system 200. Airbag system 200 includes at least one airbag 202 and an inflator 10 containing a gas generant composition 12 in accordance with the present invention, coupled to airbag 202 so as to enable fluid communication with an interior of the airbag. Airbag system 200 may also include (or be in communication with) a crash event sensor 210. Crash event sensor 210 includes a known crash sensor algorithm that signals actuation of airbag system 200 via, for example, activation of airbag inflator 10 in the event of a collision.

Referring again to FIG. 2, airbag system 200 may also be incorporated into a broader, more comprehensive vehicle occupant restraint system 180 including additional elements such as a safety belt assembly 150. FIG. 2 shows a schematic diagram of one exemplary embodiment of such a restraint system.

It should be appreciated that the airbag system 200, and more broadly, the vehicle occupant protection system 180, exemplifies but does not limit gas generating systems contemplated in accordance with the present invention.

In yet another aspect of the invention, a method of forming a gas generating composition is provided containing the following steps: (1) forming a composition containing a non-azide fuel and phase-stabilized ammonium nitrate; and

(2) at least partially integrating or mixing an additive into the composition, wherein said additive is selected from long-chain hydrocarbons, long-chain fluorocarbons, paraffinic compounds, and polymeric compounds. The paraffinic and polymeric compounds of the method may be selected from paraffin, synthetic paraffin, polytetrafluoroethylene, polyethylene, and mixtures thereof.

It should further be understood that the preceding is merely a detailed description of various embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents. 

What is claimed is:
 1. A composition comprising: a non-azide fuel selected from nitroaromatic acids and derivatives thereof; an oxidizer containing phase-stabilized ammonium nitrate; and an additive selected from paraffins, polymers, and long-chain hydrocarbons and long-chain fluorocarbons.
 2. The composition of claim 1 further comprising a secondary oxidizer selected from basic metal nitrates, and, metal and nonmetal nitrates, chlorates, perchlorates, nitrites, oxides, and peroxides.
 3. The composition of claim 1 further comprising a secondary nonazide fuel selected from carboxylic acids; amino acids; tetrazoles; triazoles; guanidines; azoamides; metal and nonmetal salts thereof; and mixtures thereof, said secondary fuel provided at about 0.1-45 weight percent.
 4. The composition of claim 1 wherein said additive is selected from paraffinic compounds.
 5. The composition of claim 1 wherein said additive is selected from polymeric compounds.
 6. The composition of claim 4 wherein said additive is selected from natural and synthetic paraffins.
 7. The composition of claim 6 wherein said additive is selected from polyethylene.
 8. A gas generating system containing the composition of claim
 1. 9. A vehicle occupant protection system containing the composition of claim
 1. 10. A method of forming a gas generant comprising the steps of: forming a composition containing a non-azide fuel and phase-stabilized ammonium nitrate; and at least partially integrating an additive into the composition, wherein said additive is selected from long-chain hydrocarbons, long-chain fluorocarbons, paraffinic compounds, and polymeric compounds.
 11. The method of claim 10 wherein said paraffinic and polymeric compounds are selected from paraffin, synthetic paraffin, polytetrafluoroethylene, polyethylene, and mixtures thereof.
 12. The method of claim 10 wherein said additive is homogeneously mixed into said composition.
 13. The method of claim 10 wherein said additive is coated about said composition.
 14. The method of claim 10 wherein said additive seals said composition.
 15. A gas generant formed by the method of claim
 10. 16. A gas generating system containing the gas generant formed by the method of claim
 10. 17. A vehicle occupant protection system containing the gas generant formed by the method of claim
 10. 18. A composition comprising: a non-azide fuel; an oxidizer containing phase-stabilized ammonium nitrate; and an additive selected from paraffin, synthetic paraffin, polyethylene, polytetrafluoroethylene, and mixtures thereof.
 19. The composition of claim 1 wherein said nitroaromatic acids and derivatives thereof are selected from dinitrosalicylic acid and ammonium dinitrosalicylic acid.
 20. The composition of claim 18 wherein said non-azide fuel is selected from dinitrosalicylic acid and ammonium dinitrosalicylic acid. 